The invention relates to a motor-vehicle drive train which contains a differential which has a differential input part and two differential output parts.
A motor-vehicle drive train of this type is known from DE 39 21 323 A1. A gear-shift transmission which can be driven by a motor-vehicle engine and can be shifted automatically or by hand drives a planetary gear mechanism. Of the planetary gear mechanism, an internally geared wheel is drivingly connected to an output shaft of the gear-shift transmission, a planet carrier is drivingly connected to a drive train leading to the rear axle of the motor vehicle, and an inner central gear is drivingly connected, via a front-axle clutch, to a drive train leading to the front axle of the motor vehicle. The planetary gear mechanism can be locked by a multi-plate locking clutch which is arranged between the planet carrier and the inner central gear and is engaged by spring means and disengaged by a pressure-medium actuated member. The front-axle clutch is engaged by a pressure-medium operated actuating member and disengaged by spring means. The planetary gear mechanism is a so-called xe2x80x9cinter-axlexe2x80x9d differentialxe2x80x9d because it distributes the propulsion power of the motor-vehicle engine in the longitudinal direction of the vehicle to the rear-wheel axle and the front-wheel axle. It is possible to select the following operating states:
1. Rear-wheel drive, in which case the locking clutch is engaged and the planetary gear mechanism is thus locked and the front-axle clutch is disengaged.
2. xe2x80x9cBalanced four-wheel drivexe2x80x9d, in which case the locking clutch is disengaged and the front-axle clutch is engaged.
3. xe2x80x9cFour-wheel drive with inter-axle lockingxe2x80x9d, in which case the locking clutch and the front-axle clutch are engaged. This is also referred to as an all-wheel drive with inter-axle locking.
The clutches are fully disengaged or engaged in each case. Friction slip operation is not envisaged as a continuous mode of operation.
A similar motor-vehicle drive train with a transfer gear for distributing the drive torque to two different vehicle axles is known from DE 195 27 484 A1.
German Utility Model G 390 20 400.0 discloses a transfer gear for motor vehicles in the case of which an output shaft for a rear vehicle axle can optionally be connected by a form-fitting clutch, and is permanently connected by a friction clutch, to an output shaft for a front vehicle axle. The friction clutch has a relatively small transmittable torque. With the form-fitting clutch disengaged, the friction clutch acts as a shock absorber for reducing gear-shifting impacts and torque load-change impacts. Its moment of friction is so small that, in the case of small differences in torque and torque shocks, the clutch provides for friction slip operation. Drive-train play is used up by it. The friction clutch is not a part of the differential.
It is known from DE 42 30 989 A1 to correct the gear-shifting times of a transfer gear or intermediate transmission in dependence on the temperature of the gear or transmission.
In a drive train, impact noises occur when the load changes, in particularly upon a changeover from thrust to traction operation or vice versa. Such noises are avoided in all-wheel drive vehicles only if there is an inter-axle lock and if this is switched on, i.e. the front-wheel drive train and the rear-wheel drive train are locked with the input train of the transfer gear by a clutch for joint torque transmission.
It is the object of the present invention to prevent, in all-wheel drive vehicles impacts or shocks in the drive train, so as to provide for a comfortable operation of vehicles independently of whether the inter-axle is locked or not.
In a motor-vehicle drive train containing a differential, which has an differential input part and two differential output parts, a friction clutch with a limited transmittable torque is arranged between two of the differential parts, so that, with a small predetermined difference in torque between the two coupled differential parts, said friction clutch can slip to accommodate torque shocks. A temperature-dependent adjusting means is provided, which reduces the clutch-engagement pressure with increasing clutch temperature.
The invention takes account essential technical basic requirements which are to be met by a torque impact damping device. The active damping moment has to act at the maximum damping value during load transfer, i.e. during a torque transmission change in the drive train from traction to thrust or from thrust to traction. In the case of particular operating states, for example in the case of cornering, in the case of different rolling radii of the vehicle wheels or in the case of slippage on a vehicle axle, different power paths are initiated in the differential, and these can activate a clutch installed in the path. This may have considerable functional disadvantages, e.g. grabbing as a result of the excitation of frictional vibration in the drive train and wear in the drive train and in the clutch if the clutch according to the prior art is designed for the maximum clutch torque. Moreover, during towing, one vehicle axle is at a standstill so that considerable loading occurs in the clutch if the latter is constantly subjected to its full engaging pressure, with the result that the clutch may become defective and fail. The invention fulfils the following basic requirements, which are to be met by a torque impact damping device:
a) The damping arrangement has to function straightforwardly by mechanical action and has to take up a minimal amount of space.
b) The damping arrangement has to function at a location where the greatest amounts of play occur during load-changes in the drive train.
c) Load-changes mean short-term loading. The damping arrangement of the invention is therefore designed specifically for short-term loading. The invention results in the damping function in the friction clutch being dissipated in the case of continuous changes in speed in the two partial drive trains, e.g. xe2x80x9cfront axle and rear axlexe2x80x9d or xe2x80x9cleft-hand and right-hand axle shaftxe2x80x9d.
d) The differential is predestined as the central installation space for the damping arrangement, or the friction clutch serving as damping arrangement, both in terms of the functional requirements and of the installation requirements in the drive train. Accordingly, in a preferred embodiment of the invention, the differential is the location where the damping friction clutch is arranged.
The invention provides, as an essential component of the friction clutch, a temperature-sensing element by means of which the contact-pressure force of the friction clutch and thus also the clutch torque can be controlled automatically. This temperature-dependent element prevents the above-mentioned disadvantages in the case of long-term loading and in the case of relatively high clutch loading (large clutch friction moments).
As the temperature-dependent element, a bimetal arrangement or a memory element arrangement (shape memory element) or some other temperature-dependent reaction element which alters its shape and/or its size depending on temperature may be used. The invention utilizes this alteration for automatically controlling the engaging pressure of the friction clutch and, if appropriate, also for fully disengaging the friction clutch.
According to a preferred embodiment of the invention, measures are provided which prevent the situation where the engaging pressure of the friction clutch is large enough for the drive train to become a xe2x80x9cdrive train with inter-axle lockingxe2x80x9d even at very low temperatures. This can be achieved by the element which can alter its shape depending on temperature and which is designed and arranged such that it only causes a temperature-dependent change in the clutch-engaging pressure above a predetermined temperature, but cannot enhance the clutch-engaging pressure below this temperature.
It is noted that a configuration of a drive train with all-wheel drive is particularly advantageous because only a small amount of space is required.
The invention will be described herein below with reference to the accompanying drawings on the basis of preferred embodiments.